Synthesis,Crystal Structure and Fluorescent Properties of New Layered Thiophosphate Cs2Ga3PS8①

CHEN Wen-F LIU Bin-Wen JIANG Xio-Ming② GUO Guo-Cong②

a (State Key Laboratory of Structural Chemistry,Fujian Institute of Research on the Structure of Matter,Chinese Academy of Sciences,Fuzhou 350002,China)

b (Uniνersity of Chinese Academy of Sciences,Beijing 100190,China)

ABSTRACT A new quaternary metal thiophosphate,Cs2Ga3PS8,in triclinic Pspace group has been successfully synthesized by a reactive-flux method.Its structural framework is derived from well-known AMIIIMIVQ4 (A=alkali metal;MIII=Al,Ga,In;MIV=Si,Ge,Sn;Q=S,Se) system and composed of two-dimensional layers separated by Cs+.The compound exhibits a wide band gap of 3.08 eV and congruent-melting behavior with melt point of 645 ℃.Interestingly,Cs2Ga3PS8 exhibits a broad photoluminescent emission band at 420 nm upon excitation at 295 nm.Moreover,electronic structure calculations indicate that Cs2Ga3PS8 is a direct band gap compound and its luminescent process can be mainly ascribed to electron transfer from the S-3p and Ga-4p states to S-3p and P-3p.

Keywords:chalcogenide,thiophosphate,crystal structure,solid-state phase,synthesis;DOI:10.14102/j.cnki.0254-5861.2011-3135

1 INTRODUCTION

In the past decades,many achievements have been made in exploring functional materials in chalcogenides,which can be used as nonlinear optics,electro optics,superionic conductors,and pyroelectrics[1-10].As an important subgroup of chalcogenide,thiophosphates exhibit rich structural diversity as well as unique physical properties,and have received broad attention[11-15].Thiophosphates are typically composed of tetrahedral [PQ4]3-(Q=S,Se,Te) and ethane-like [P2Q6]4-units,the combination of which could further generate more complex building blocks such as [P2Se6]4-[16],[P2Se9]4-[17],and infinite chains like [P2Se6]2-[18],[PSe6]-[19],[P5Se10]5-[20].Moreover,discrete [PxQy]n-fragments can be assembled with other metals to form a variety of extended frameworks with fascinating properties.For example,A4GeP4Se12(A=K,Rb,Cs) are excellent IR NLO materials exhibiting large second-harmonic-generation effect which is～30 times that of bench AgGaSe2at 730 nm[21].AZrPS6(A=K,Rb,Cs) are unique examples of stable inorganic semiconductors with band gap emission very attractive for technological applications[22].Rb4Sn5P4Se10is a semimetallic seleno-phosphate and displays high conductivity of 51 S/cm at 300 K[23].Li10SnP2S12is an affordable lithium superionic con-ductor with very high values of 7 mS/cm for the grain conduc-tivity[24].Although many thiophosphates have been found,investigations on thiophosphates containing Ga are rare.During our attempts to explore A-Ga-P-Q system,a new phase,Cs2Ga3PS8(1),has been synthesized.Herein,the syntheses,structures,and thermal and optical properties of 1 are presen-ted.Interestingly,the compound exhibits a broad photo-luminescent emission band at 420 nm.To gain further in-sights on its luminescent properties,the calculations of elec-tronic band structure and density of states were performed.

2 EXPERIMENTAL

2.1 Syntheses

The following reagents were used as obtained:Ba metal (99.9%),Ga metal (99.99%),P powder (99.99%),S powder (99.99%),and CsCl powder (99.99%).All operations were handled under an Ar atmosphere in a glove box.The title compound was synthesized by the stoichiometric mixture of Ba,Ga,P,S,and CsCl with total mass of 500 mg in a molar ratio of 1:3:1:8:2.The mixture was loaded into quartz tubes and then flame-sealed.The tubes were placed into a computer-controlled furnace,heated to 750 ℃ over 24 hours,subsequently dwelled for 4 days,and finally cooled down to room temperature at 3 ℃/hour.After the products were washed with deionized water and dried with methanol,lamellar colorless single crystals of 1 were observed,and the samples for further property measurements were obtained by hand picking under a microscope.

2.2 Single-crystal X-ray diffraction

Single-crystal X-ray diffraction measurement was per-formed on a Rigaku Pilatus CCD diffractometer using a graphite-monochromated Mo-Kαradiation (λ=0.71073 ?) at 293 K.The intensity dataset of the title compound was collected using anω-scan technique and reduced using the CrysAlisPro[25].The structure was solved by direct methods and refined with full-matrix least-squares methods onF2with anisotropic thermal parameters for all atoms[26].

2.3 X-ray powder diffraction

Powder X-ray diffraction (XRD) data were recorded on an automated Rigaku MiniFlex II X-ray diffractometer equipped with a diffracted monochromator set for Cu-Kαradiation (λ=1.54057 ?),operating at 30 kV and 40 mA.The observed powder pattern of the title compound was well-suited to the simulated one (Fig.S1b).

2.4 Elemental analysis

Selected crystals were fixed on the sample platform and analyzed by energy dispersive analyses X-ray spectroscopy (EDX) by using an EDX-equipped Hitachi S-3500 SEM spectrometer.Energy dispersive spectroscopy (EDS) analysis of the crystals of the title compound confirmed the presence of Cs/Ga/P/S with a molar ratio of 2.0/2.9/1.1/7.8,which is close to that determined from the single-crystal X-ray diffraction analysis (Fig.S1a).

2.5 UV-Vis diffuse reflectance spectroscopy

Optical diffuse reflectance measurement was made to measure the band gap of the title compound by Perkin-Elmer Lambda 900 UV-Vis spectrophotometer accompanied with an integrating sphere attachment,with BaSO4used as a reference.Absorption spectrum was calculated from the reflection spectrum using the Kubelka-Munk formula:α/S=(1 -R)2/2R[27],in whichαis the absorption coefficient,Sthe scattering coefficient,andRthe reflectance.

2.6 Photoluminescence

The photoluminescence (PL) measurement of 1 was conducted on a single-grating Edinburgh EI920 fluorescence spectrometer equipped with a 450 W Xe lamp and a PMT detector.

2.7 Thermal analysis

Thermal properties of the title compound were measured by differential scanning calorimetry (DSC) with a TGA/DSC Mettler Toledo thermal analyzer.Polycrystalline sample (approximately 10 mg) was put into a quartz tube,then evacuated to～10-4Torr and sealed.Finally,the tube experienced a heating/cooling cycle at a rate of 10 ℃/min.

2.8 Electronic structure calculation

The electronic band structure and density of state (DOS) of 1 were calculated by the CASTEP code[28]on the basis of density functional theory (DFT)[29],using a plane-wave expansion of the wave functions and an ultra-soft pseudo potential.The orbital electrons of Cs 5s25p66s2,Ga 3d104s24p1and S 3s23p4were treated as valence electrons.A plane-wave cutoff energy was set to be 295 eV with a grid of Monkhorst-Packk-points of 4×4×2.

3 DISCUSSION

3.1 Structure description

Compound 1 crystallizes in monoclinic space group ofP(No.2) witha=7.22730(10),b=7.64670(10),c=14.2671(3) ?,α=91.005(2),β=91.146(2),γ=106.016(2)°,V=757.50(2) ?3andZ=2.The asymmetric unit is depicted in Fig.1a.There are two crystallographically independent Cs atoms,two Ga atoms,eight S atoms,and two mixed positions with equal occupancy of Ga and P.The title compound exhibits a two-dimensional layer structure (Fig.2a).All Ga and P atoms are tetrahedrally coordinated by S atoms to form GaS4and (Ga/P)S4tetrahedra.GaS4tetrahedra share two corners with each other to form 13 tetrahedra chains extending along theadirection,which are further bridged by (Ga/P)S4tetrahedra dimers alternately,forming a [Ga3PS8]2-layer in theacplane (Fig.2b).The (Ga/P)S4tetrahedra dimers are constructed by two edge-shared (Ga/P)S4tetrahedra.The counter Cs+are embedded between [Ga3PS8]2-layers.

Fig.1.Coordination environments of Ga and P atoms (a),and ionic interactions around Cs atoms (b) in the asymmetric unit of 1

Fig.2.(a) Crystal structure of 1 viewed along the a axis.(b) A [Ga3PS8]2- layer.Green and purple tetrahedra represent (Ga/P)S4 and GaS4 units,respectively.Black balls in (a) are Cs atoms

Compound 1 belongs to Cs2M3IIIMVQ8(Q=S,Se,Te) family (type-I)[30],which can be derived from AMIIIMIVQ4family (type-II)[31,32]by replacing all MIVatoms with equal amounts of MIIIand MVatoms.The modification of AMIIIMIVQ4family can also lead to A2MIIMIV3Q8(Q=S,Se,Te) family (type-III) via the substitution of two MIIIatoms by one MIIand one MIVatoms[33-35].The structures of type-I,II and III compounds are similar,but they exhibit different structure disorders of tetrahedrally coordinated centers.In type-II compounds,the trivalent and tetravalent metal ions are disordered over all tetrahedral sites.Type-III family of compounds is completely ordered,whereas in the type-I compounds,all MVand partial MIIIpositions are disordered.The flexible substitution behavior of AMIIIMIVQ4family makes it a good platform for exploring new materials with rich structure features and physical properties[31,32].

As listed in Table S2,Ga-S distances of fully occupied GaS4tetrahedra in 1 are in the range of 2.2484～2.3361 ?,which are close to those inβ-LaGaS3(2.194～2.325 ?)[36]and SnGa4S7(2.214～2.337 ?)[37].In (Ga/P)S4tetrahedra,the Ga/P-S distances range from 2.1087 to 2.2309 ?,which are between the typical P-S and Ga-S bond lengths.Two crystallographically independent Cs atoms are surrounded by nine and eleven S atoms,respectively,with ionic interactions.The Cs-S distances in the range of 3.469～4.118 ? (Fig.1b) are consistent with those in Cs[Lu7S11][38].

3.2 Experimental band gap and photoluminescent spectra

The UV-Visible-NIR diffuse reflectance spectrum of 1 exhibits obvious absorption edge and the band gap is estimated to 3.05 eV (Fig.3a),which is consistent with its colorless feature.The band gap of 1 is comparable to those of some other thiophosphates,such as KAg2[PS4] (3.02 eV)[39]and K4GeP4S12(3.0 eV)[21].The photoluminescent spectra of 1 were studied in the solid state at room temperature,and its excitation and emission spectra are plotted in Fig.3b.Compound 1 exhibits a broad photoluminescent emission band at 420 nm upon excitation at 295 nm.

Fig.3.(a) UV-Vis diffuse reflectance spectrum and (b) excitation and emission spectra of 1

3.3 Differential thermal analysis

The differential scanning calorimetry (DSC) was used to examine the thermal properties of 1 (Fig.4),which showed that the compound exhibits a broad endothermic peak on the heating curve,that is,crystals of 1 melt at 645 ℃.Correspondingly,there is an exothermic peak at 626 ℃ for crystallization during the cooling process.

3.4 Electronic structure calculation

To better understand the optical properties,theoretical calculations including electronic band structures and partial density of states (PDOS) of 1 are calculated by DFT.The calculated electronic band structure is plotted in Fig.5a,indicating a direct band gap of 1.839 eV.The PDOS (Fig.5b) shows that the conductive band (CB) close to the Fermi level is mostly composed of S-3pand P-3pstates,as well as a small portion of P-3sstate.While the valence band (VB) from -4.0 eV to the Fermi level originates predominately from S-3pand Ga-4pstates.The contributions of Cs atom states to bands from -6 to 9 eV are negligible,so luminescent properties of 1 can be mainly ascribed to electron transfer from S-3pand Ga-4pstates to the S-3pand P-3pones.

Fig.4.DSC curves of 1

Fig.5.Electronic band structure (a) and the partial density of states (b) of 1

4 CONCLUSION

In summary,a new phase,Cs2Ga3PS8,in triclinic space group ofPhas been successfully synthesized by high-temperature reactant flux method.Its structure is built from 2Dinfinite2∞[Ga3PS8]2-layers,separated by Cs+.UV-vis-NIR spectroscopy measurement indicated that Cs2Ga3PS8shows a wide band gap of 3.08 eV.The melting point of this compound is 645 ℃.Cs2Ga3PS8exhibits a broad photoluminescent emission band at 420 nm upon excitation at 295 nm.Theoretical calculation of electronic band structure indicated that fluorescent properties of Cs2Ga3PS8origin charge transfer from S-3pand Ga-4pstates to S-3pand P-3pstates.